Field of the Invention and Related Art Statement
This invention relates to an apparatus for measuring the optical characteristics of a substrate for an optical recording medium adapted to investigate optical characteristics such as a light leakage to be suppressed in a crossed Nicol state set in a recording or reproducing optical system.
Recently, there has come to be noted an optical information recording and reproducing apparatus which can record information at a high density in an optical recording medium by condensing and projecting light beams onto the recording medium and can read out (reproduce) at a high speed the recorded information written in the recording medium by receiving, with a photodetector, the light returning from this recording medium.
The above mentioned recording medium is a photomagnetic disc in which is formed a magnetic recording layer wherein light beams passing through a substrate such as acryl resin as a PMMA, are condensed and projected onto this substrate and the polarized plane of the returning light rotates in response to the magnetizing direction of the part forming the recording layer or a photodisc in which information is recorded in pit rows which reflect light in different amounts.
As disclosed, for example, in the publication of Japanese Patent Laid Open No. 74701/1982, the above mentioned acryl resin has high optical characteristics but has a defect that it is as high in the hygroscopicity so that the recording medium surface will deflect.
Therefore, it is thought to be effective to use for a substrate a resin such as a polycarbonate (abbreviated as PC hereinafter) resin which is hard to deflect, high in form stability and also high in mechanical strength.
In using a resin such as the above mentioned PC resin for a substrate, it is necessary to know its optical characteristics. For example, in case its refractive index is large, the optical distance per unit thickness will become longer and therefore the thickness of the substrate can not be made too large. Therefore, it is particularly necessary to investigate the refractive index of the material to be used for PG,4 the substrate and, as the refractive index may vary depending on the method of forming the substrate, it is desirable to measure the refractive index on the substrate form made by an actual molding method.
FIG. 1 shows a prior art example of an apparatus 1 for measuring a double refractive index for a substrate of a disc-shaped recording medium.
That is to say, a random polarized laser beam of an He-Ne laser 2 is made a predetermined linear polarized light beam by passing through a polarizer 3 such as a Glan-Thompson prism (abbreviated as GTP) and is then projected onto a substrate 4 as a medium to be measured. The light beam having passed through this substrate 4 passes through a phase compensating plate 5 of Babinet-Soleil arranged to be opposed to the above mentioned polarizer 3 with the substrate 4 held between them, then passes through an analyzer 6 such as a GTP set in an extinction position (crossed Nicol) so as to pass the polarized light intersecting at right angles with the above mentioned polarizer 3 and is received by a light receiving element 7. With the polarized beam incident upon the above mentioned substrate 4, if the substrate 4 is of a monoaxial (crystal) characteristic by which the optical axis is vertical to the substrate plane, even if the polarizer 3 is rotated to vary the polarizing direction, no phase difference (ellipticizing) will be produced in the substrate 4 but, in case the optical axis is within the substrate, a phase difference will be produced in response to the angle formed by the polarizing direction with the optical axis and, even in the case of a biaxial (crystal) characteristic, when the polarizing direction of the polarized beam is varied, a phase difference will be produced in the substrate. Therefore, of a left rotatory plate 5a and wedge-shaped right rotatory plates 5b of the phase compensating plate 5, for example, if the right rotatory members 5b are moved in the vertical direction (within the paper surface) to uniformly vary the thickness of the plate by the wedges and thereby the above mentioned phase difference is extinguished, the light passing through the analyzer 6 in the crossed Nicol state (extinction position state) for the polarizer 3 will be able to be extinguished and the signal output of the light receiving element 7 will become a minimum. The double refractive index within the plane of the substrate 4 can be measured from the displacement of the above mentioned phase compensating plate 5.
With the above mentioned conventional measuring method, the double refractive index in the thickness direction of the substrate can not be known at all. Therefore, in case it is used for the substrate of an optical recording medium, it will be insufficient. That is to say, in case the light is projected onto the recording layer through the substrate of the recording medium, a parallel light beam will be focused in the form of a spot and will be radiated, the light condensing angle or the number of apertures N.A. will be considerably large and the position of the substrate surface will be held in a defocused state and will be hard to be influenced by dust or the like. It shall be explained in the following that, when the light beam is thus condensed, in case the substrate is of an optical material showing a double refraction, the refractive index in the thickness direction will influence the light beam passing through the substrate.
In case an injection-molded PC plate is used for the above mentioned substrate, this substrate will show double refraction such as of a monoaxial crystal and will have an optical axis in the direction vertical to the substrate plane in most cases. The refractive index (no) for the normal ray and refractive index (ne) for the abnormal ray are different from each other.
Therefore, the linear polarized light incident upon this substrate as inclined to the optical axis (the direction vertical to the substrate plane) will produce a phase difference due to the double refraction unless the angle formed by the incident plane with the polarizing direction is a specific angle and, as a result, an ellipticization (a linear polarized light becoming an elliptic polarized light) will be produced.
FIG. 2 is an explanatory view showing the manner of throttling a laser beam 14 into a part of a substrate 12 forming a disc 11 of an objective so as to be radiated in the form of a spot. In the drawing, only a part of the disc 11 is shown.
The laser beam 14 is a linear polarized light whose polarizing direction intersects at right angles with the radial direction 16 of the substrate 12 as indicated by the arrow 15 and includes a beam portion (S polarized light) 21 incident vertically to the polarizing direction, a beam portion (P polarized light) 22 incident parallelly with the polarizing direction and beam portions 23 and 24 incident as inclined, for example, by 45 degrees respectively to these beam portions 21 and 22. These beam portions 23 and 24 become a polarized light including both components of S polarized light and P polarized light.
On the other hand, the refractive indices for the S polarized light and P polarized light incident upon the substrate 12 as inclined by an angle .theta.i to the optical axis (direction vertical to the substrate plane) indicated by the arrow 12a in FIG. 2 are determined as in the following.
FIG. 3 is an explanatory view showing the relation between the incident angle .theta.i and refractive index of the light incident upon the substrate 12. As described above, the injection-molded PC substrate 12 shows a substantially monoaxially crystalline characteristic and two of the main refractive indices n.sub.1, n.sub.2 and n.sub.3 are equal. Therefore, if a refractive index ellipsoid is indicated by selecting coordinate axes so that n.sub.1 =n.sub.2 and the Z axis direction may be n.sub.3, the optical axis 12a will coincide with the Z axis.
Here, the refractive index n' for the S polarized light incident as inclined by the angle .theta.i to the optical axis (direction verical to the substrate plane) 12 and the refractive index n" for the P polarized light are represented respectively by the short axis 26a and long axis 26b of the vertically cut section (ellipse 26) of the light 25 after the incidence. That is to say, if the angle made by the light 25 after the incidentce with the optical axis 12a is represented by .theta.t, ##EQU1## Here, EQU sin .theta.t=(1/n') sin .theta.i
Therefore, the beam portion 21 of the S polarized light incident upon this substrate 12 and the beam portion 22 of the P polarized light remain linear polarized light but, for example, the beam portions 23 and 24 incident as inclined by 45 degrees to the above mentioned beam portions 21 and 22 are polarized light including both components of the S polarized light and P polarized light. Therefore a phase difference will be produced between the S polarized light component and P polarized light component and the linear polarized light will become elliptic polarized light. If the thickness of the substrate 12 is represented by d and the wave length of the laser beam is represented by .lambda., this phase difference .delta..sub.s-p will be represented by EQU .delta..sub.s-p =(2.pi./.lambda.).times.(n'-n").times.(d/cos .theta.t)(3)
Therefore, the larger the thickness d and incident angle .theta.i of the substrate, the larger the phase differene .delta..sub.s-p.
FIG. 4 is a sectioned view of a beam which was incident upon the objective by the linear polarized light whose polarizing direction is represented by the reference numeral 27, was reflected by the disc 11 and again passed through the objective. The nearer to the peripheral edge side of the beam, that is, the larger the opening, the larger the beam incident upon the substrate 12 in the incident angle .theta.i and phase difference .delta..sub.s-p. Where the orientation angle (the angle made by the polarizing direction with the incident angle) corresponds to 45 degrees (that is, such as 45 and 135 degrees), that is, at the reference numerals 28a, 28b, 28c and 28d, the ellipticization is a maximum.
Thus, in case the substrate 12 shows a double refraction, even if the double refraction is of a monoaxial characteristic, by the refractive index for the thickness direction, the linear polarized light will become an elliptic light having a polarized component at right angles with the linear polarizing direction.
Therefore, in case the substrate is used as a substrate, for example, for a photomagnetic disc, when a linear polarized light is radiated, the polarizing direction of the returning light will be rotated by a minute angle in response to the direction of magnetization but, even if an analyzer is set to transmit only the rotated polarized light component, the light beam having passed through the substrate will be ellipticized and therefore other light beams than the inherent signal component will also pass through this analyzer and these leaking lights will mix in with the signal. Also, there will be produced a signal component intercepted by the analyzer due to the ellipticization. Therefore, the C/N (carrier to noise ratio) will be reduced.
There is extensively used an optical system wherein, in case the substrate is used not only as a substrate for a photomagnetic disc but also as a substrate for a photodisc in which recorded information is reproduced from the difference in the reflected light amount, a light beam obtained by making a linear polarized light pass through a polarized beam splitter, a circular polarized light by using a .lambda./4-plate is radiated, this returning light is again made a linear polarized light in the polarizing direction intersecting at right angles with the above mentioned linear polarizing direction by the .lambda./4 plate and this linear polarized light is efficiently branched on the information photodetector side by the above mentioned polarized beam splitter. In such a case, too, the substrate (having a value different from the refractive index in the substrate plane direction) will be made elliptic due to the refractive index in the thickness direction, the light will not be efficiently branched and the C/N value will be reduced.
As the ellipticization by the above mentioned double refraction is different in the respective positions of the circular light beam, it is necessary to evaluate to what extent the C/N is influenced in the actual used condition. In such a case, the C/N can be evaluated in principle by determining the refractive index in the thickness direction but the influence is different depending on the incident angle and the like. In case the condition in the evaluation is different from the actual used condition, in the case of applying it to the actual used condition, the quantitative determination will be difficult. Therefore, if there is a simple means of evaluating the C/N in the state approximating the actual used condition, it will be very convenient.